The economical or strategic importance of oil, is evident, therefore the possibility of increasing the production, as well as the usable reservoirs, is extremely attractive. However, new oil fields have not been found, and oil and gas reservoirs have been reduced in the last three years. In addition, the cost reduction is a constant existing need in all industries in order to increase the competitivity and profitability of the companies. In the case of oil production, the cost reduction increases the economically exploitable reservoirs, since oil is extracted provided the income is higher than the operative costs. In Argentina, where there are many mature oil fields exploited by means of secondary recovery where the average cut (water ratio in the extracted fluids) is over 90%, the idea of a productive alternative for reducing costs is particularly attractive.
Typically, once an underground formation capable of containing oil and/or gas is located, a well is drilled, and depending on the type of ground to be passed through and the final depth to be reached, it may begin with a diameter of about 12.¼″ (311,15 mm) in the first 200/300 mts, of 8.½″ (215,9 mm) at a higher depth, capable of reaching a depth of 400 mts up to 4500 mts or more. The greater diameter allows placing a steel line (guiding line or security line) which will be fixed to the ground by means of a forced introduction of cement in the annular space between the tube and the ground. The blow out prevention (BOP) valve will be located above said tubing during the drilling with the smaller diameter. Following this drilling of smaller diameter, it is required to introduce a tubular steel lining, namely casing of about 5.½″ (139,7 mm) diameter extending along the whole well bore. As well as with the security line, the forced introduction of cement in the annular space between the casing and the walls of the borehole from the bottom to a height beyond the areas of interest will allow to fix the casing once the cement is forged.
Following the above operation, punctures are made at pre-selected depths, in accordance with the nature of the reservoir, which go through both the casing wall and the cement sheath, allowing a free access of production fluids from the formation to the well bore.
In some regions, the pressure of the reservoir fluids itself is sufficient to allow the natural lift of the fluids to the surface, rendering a flowing well. However, reservoirs are generally not eruptive, being necessary to extract the fluids entered into the well in an artificial manner by means of a pumping system.
A conventional reciprocating pumping well includes, in addition to the casing, the production line or tubing within which the produced fluids are passed from the bottom to the surface. At the bottom of the well and anchored in the production tubing, there is a reciprocating axial pump of barrel-rod type. This pump is mechanically actuated by an oscillating lever pivotally assembled on the surface, connecting in one end to a driving source and in the other to a series of solid steel rods connected each other to form a string which extends within the well, being connected by its lower end to the mobile part of the deep well pump and imparting it the reciprocating movement of the oscillating lever. In this way, the pumped fluids ascend to the surface trough the annular space defined between the production tubing and the rod string.
The solid rods movement within the production tubing involves a frictional contact between both, thus producing the rod and/or production tubing break and damages to the system. This problem increases in deviated or crooked holes. The high number of interventions in holes due to this problem generates high maintenance costs and increased production losses.
In such system, the production tubing cost is a significant part of the total investment.
The conventional pumping system with production tubing provides low pumping efficiency due to its stretching and shortening, which occurs with the change of direction of the rod string between ascending and descending movement.
Another relevant disadvantage of the system is that, whenever a service on the pump is needed in the fixed tubing system (when due to its size the pump is fixed to the production tubing) it is necessary to remove the rod string and the production tubing, thus increasing the hole intervention and interruption times, with the corresponding rise in costs, and loss of production.
Among the attempts to reduce costs, we can mention some patents that tend to reduce the rods' weight in order to require less energy for its operation, although the energy required in this system is not directly related to the loads, since these regenerate energy during the ascending movement. Among these patents, we can mention the following:
AR patent No. 230316 refers to a pumping rod, essentially made of fiber glass, with a significantly lower weight.
AR patent No. 234862 suggests the replacement of the rod string with a fiberglass rod string with no mutual contact, in order to achieve a lower weight of the rods.
Other attempts have proposed the use of continuous coiled tubing in order to replace the solid rod string. Among them, we can mention the following:
U.S. Pat. No. 5,667,369 (H. Cholet) proposes the replacement of the sucker rods with a continuous coiled tubing that has a PCP type pump rotor (progressive cavity pump) bound to its bottom end, and wherein the corresponding stator is bound to the base of a production tubing. In this case, the pump activation is carried out by means of a rotating movement of the continuous tubing, lower in weight and easy to handle, but the use of the production tubing is maintained.
The AR published application No. 0010430 (YPF S.A.), (U.S. Pat. No. 6,186,238) proposes to replace the combination of solid pumping rods and production tubing in the conventional reciprocating pumping system by a continuous coiled tubing. Among its advantages, this system has a fast downloading handling as well as a fast pump extraction. The great flexibility helps its coiling, however, it reduces the strength to absorb compression efforts and buckling during the ascending run, thus increasing the possibility of friction against casing walls, particularly in deep wells. Consequently, this system is successfully applied in shallow and low production wells. In addition, once broken, the continuous tubing must be bound by welding, thus reducing the resistance against weariness in said area. This reduction is enhanced by the amount of splices, considerably reducing the life of the continuous tubing that may derive in a total change of it. The continuous tubing system does not allow the use of vulcanized centralizers on the tubing body; only bayonet centralizers may be used, with no satisfactory results.
U.S. Pat. No. 4,476,923 (Walling), describes a coiled, composite tubing that allows the effluents to be conducted through its internal cavity. Such composite tubing supports, in its bottom end, a pump mechanically actuated by means of an electrical engine housed in the same deep well pump casing. The engine is electrically actuated from the surface by means of conductors extending along and across the composite tubing sheaths. Therefore, the composite tubing comprises a complex sequence of sheaths and wrappings made of different materials able to provide the resistance required to support this particular pumping system.
In U.S. Pat. No. 4,089,626, the hollow rods are used to inject chemical products to the bottom of the well. The possibility that the well fluids may be produced from its interior is not mentioned. Consequently, the production line (tubing) has not been removed in this patent.
U.S. Pat. No. 4,948,003 describes a method of taking crude samples wherein the hollow rods are used to inject chemicals, such as surfactants, that increase the viscous fluids mobility. However, a conventional solid rod tubing system is also used in this patent.
The Chinese Patent Application 95-104622.5 describes a production system with hollow rods that uses a flexible tube (hose) in the well head for the derivation of the fluids to the production tubing, accompanying the reciprocating movement of the rod string to the production tubing. This arrangement increases the environmental pollution risks due to the possibility of high pressure, several bent parts and harsh environments that may produce a malfunction in the flexible tube.